Allow for Layout Builder sections to be given administrative labels

Problem/Motivation

Sections are currently given numbers by their order. Section 1, Section 2, etc.
As a more complex layout can be built by combining many simple layouts, it would be nice to be able to give meaningful names to these sections.

These would only be shown when editing the layout, never on the front end.

Proposed resolution

Add ability to give sections names.
For now, this will only be used when the numeric version of the section is display: the remove section and configure section forms, and the visually-hidden parts of the UI used by screen readers.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet